Thermoplastic polymers (thermoplastics) are one of the major classes of polymer material. A solid thermoplastic polymer can typically be heated to soften and ultimately melt it, and then cooled to return it to its solid state. These temperature-induced changes are mostly fully-reversible. Thermoplastics can be divided into two broad groups: “amorphous thermoplastics” and “semi-crystalline thermoplastics”. In solid amorphous thermoplastics all of the polymer chains are arranged in a random or disordered state: none of the polymer chains are arranged in a crystalline structure. In solid semi-crystalline thermoplastics the structure is mixed: in some portions of the material the polymer chains are arranged in a ordered crystalline structure, and in some portions the chains are in an amorphous state. “Crystalline thermoplastics” have a higher proportion of crystallinity, but still have some amorphous portions. For the purpose of this discussion, crystalline thermoplastics will be grouped with semi-crystalline thermoplastics, and the term “semi-crystalline thermoplastic” will also include “crystalline thermoplastic”. In addition for the purpose of this discussion, “amorphous polymers” or “amorphous thermoplastics” and “semi-crystalline polymers” or “semi-crystalline thermoplastics” refer to types of thermoplastic polymer material, rather than to the local microstructure of any portion of thermoplastic polymer material.
Amorphous thermoplastics are characterised by a glass transition temperature (Tg) above which, with further heating, progressive softening occurs. At temperatures substantially higher than the glass transition temperature these thermoplastics behave like a high viscosity liquid. The service temperature of amorphous thermoplastics is below their glass transition temperature. They are also as a class generally susceptible to chemical attack and fluid absorption.
Semi-crystalline thermoplastics have a distinctive melting temperature (Tm), above which the material melts and behaves as a liquid. With further increases in temperature the viscosity falls off quickly. Semi-crystalline thermoplastics also have a characteristic glass transition temperature, often well below the melting temperature, due to their amorphous portions. Whether the semi-crystalline thermoplastic is above or below its glass transition temperature also influences some properties of these thermoplastics. However semicrystalline thermoplastics can often be used at service temperatures well above their glass transition temperature, because their crystalline portions are very rigid. Typically, semi-crystalline thermoplastics absorb less fluid than amorphous materials.
In both amorphous thermoplastics and semi-crystalline thermoplastics, changes induced by heating or cooling are normally fully reversible, unless the decomposition temperature, typically much higher than either the glass transition temperature or the melting temperature, is exceeded.
Thermosetting polymers are a second class of polymer that includes epoxide (often called epoxy), bismaleimide and vinyl ester polymers. An addition-polymerisation thermosetting polymer such as epoxy prior to curing consists of (as a minimum) a resin (monomer) and a hardener, which react together to produce a cross-linked polymer. Prior to curing, the monomer and hardener are normally in a liquid form, although their viscosities may be very high. Curing may be designed to occur at room temperature or higher temperatures, typically up to 180° C. for epoxies. During curing the monomer and hardener react, and the viscosity of the mixture increases until it becomes a cross-linked solid polymer. This change is not reversible. After curing the thermosetting polymer also has a characteristic glass transition temperature (typically slightly greater than the recommended curing temperature for epoxies) above which considerable softening of the thermosetting polymer occurs, and the thermosetting polymer behaves like a rubber. (Further heating does not melt the polymer—instead it typically starts to decompose at higher temperatures.) This is critical for subsequent processing such as high-temperature joining of components that contain a thermosetting polymer (e.g. a carbon fibre/epoxy composite), as dimensional distortion of the components can occur when the glass transition temperature of the thermosetting polymer is approached or exceeded.
Composite materials are a class of material which consist of at least two constituent materials, intimately joined together, which together behave as one material with different, usually superior, properties to either of the constituent materials. Polymer composites consist of polymers, either thermosetting or thermoplastic, reinforced by fibre or particulate reinforcement. Well-known polymer composites include glass fibre reinforced polyester resin, and carbon fibre reinforced epoxy. Both these use thermosetting polymers as the matrix, and are therefore often called thermosetting composites.
One major difference between thermoplastic and thermosetting polymers is that thermoplastics can be melted and resolidified by raising and lowering temperature, whereas thermosetting polymers cannot. This characteristic has been utilised for the welding of thermoplastics and thermoplastic composites, whereas thermosetting polymers or thermosetting composites cannot be joined simply in this fashion.
Thermosetting polymer components with thermoplastic surfaces are attractive, some advantages being enabling the enhanced surface properties of the thermoplastic and potentially for welding of similarly surfaced components. Normally this would be done by an adhesive bonding process. In an adhesive bonding process, the adhesive is brought into contact with the component, must flow and wet the component, and is then solidified in situ. It is quite common to make an adhesive joint between a thermosetting polymer and a thermoplastic polymer. In the most common method an uncured thermosetting polymer such as an epoxy is used as the adhesive, brought into contact with a solid thermoplastic polymer, and subsequently cured. This could be done as part of the process to cure a thermosetting composite component. Alternatively, a thermoplastic polymer can be used as the adhesive, by heating it to melt it and bringing it into contact with a cured thermosetting component. The thermoplastic resin is subsequently cooled.
In both these situations, it is difficult to generate strong adhesive bonds between the thermosetting polymer and thermoplastic polymer. Where the thermoplastic is used as the adhesive, the joint relies on weak secondary chemical bonds and is therefore itself weak. Where an uncured thermosetting polymer functions as the adhesive, on a thermoplastic surface, there are generally few sites for the formation of the higher strength primary chemical bonds. These bonds can be encouraged by surface treatment of the thermoplastic, either with a chemical agent or by physical means such as plasma treatment. This can be time-consuming and expensive, may not provide sufficiently high strength or reliability for a critical application such as the assembly of aircraft components, and may still be subject to chemical attack.
However, a better method of achieving high strength attachment between thermosetting and thermoplastic polymers is by the formation of a semi-interpenetrating polymer network. These provide a form of mechanical interlock between the polymer chains of different polymers (in this case thermosetting and thermoplastic polymers) by having the chains of one polymer interpenetrating the other.
Previously, amorphous thermoplastic materials have been joined to thermosetting composites by formation of an interpenetrating polymer network during the curing of the thermosetting composite by encouraging the liquid, uncured components (monomer and hardener) of the thermosetting polymer to migrate into the amorphous thermoplastic before the thermosetting polymer cures, utilising the low solvent resistance of the amorphous thermoplastic. This migration into the amorphous thermoplastic would normally occur below the glass transition temperature of the thermoplastic, at which condition the material is solid. This effectively gives the cured thermosetting composite a thermoplastic surface, with the ability to join to a similarly-surfaced material under increased temperature and some joining pressure.
The above process, and the amorphous thermoplastic required for it, has several disadvantages. Firstly, the low solvent resistance required for the amorphous thermoplastic used in this process means that the surface and any joint formed from this surface is likely to be susceptible to solvent attack. Secondly, with this process there is an inherent difficulty in attempting to select materials which will allow easy and efficient surfacing and welding processes as well as provide a high service temperature in the subsequent welded joint. In order to join two components with amorphous thermoplastic surfaces, the glass transition temperature of the thermoplastic has to be substantially exceeded, possibly by at least 50° C., to obtain a high quality joint in a reasonable time. As a result, the glass transition temperature of the underlying thermosetting polymer is typically exceeded, which leads to reduced stiffness and dimensional instability of the component. Dimensional change of the components is likely, unless adequate tooling is used to support the component at the joining temperature, especially as high pressures may need to be applied to the joint in order to obtain good contact and sufficient flow for consistent high-quality joints. If a sufficiently high temperature is required for the joining process, degradation of the thermosetting polymer or thermosetting composite can also occur. If a high-temperature amorphous thermoplastic is chosen as the surfacing/welding material in order to boost the service temperature of the weld, the surfacing and welding processes must in general be conducted at higher temperatures, risking dimensional change or degradation of the thermosetting composite. If a lower-temperature amorphous thermoplastic is chosen for easy surfacing and welding, the service temperature is likely to be unacceptably low. Finally, joining to a high-temperature amorphous thermoplastic often requires special long and/or complex cure cycles, for example cure cycles including dwell times below the normal curing temperature, in order to have the thermosetting monomer and hardener penetrate to a depth sufficient for adhesive strength. This may add many hours to the manufacturing time of a component, resulting in increased costs of production.
U.S. Pat. No. 5,643,390 describes a process of bonding a thermoplastic layer to a thermoset composite. The described method involves “selecting a thermoplastic material and a thermosetting monomer wherein said thermosetting monomer has similar solubility parameters to that of said thermoplastic material”. “Similar solubility parameters” is defined in terms of Hildebrand solubility theory, which is not suitable for the description of polymers with substantial polar and/or hydrogen bonding forces.
This US patent is directed to the use of amorphous thermoplastics. The patent advises that the mobility of penetrants in semi-crystalline polymers is extremely small, and this prevents the formation of an interpenetrating network to provide adhesive strength. There is also no discussion of the compatibility of semi-crystalline thermoplastic polymers.
In contrast, the present invention is a process which utilises semi-crystalline polymers, advantageously allowing easier surfacing and subsequent welding, and not compromising the solvent resistance of the subsequent welded joint.
U.S. Pat. No. 5,667,881 describes a method for fabricating an integral thermoset/thermoplastic composite joint. The described method requires that the thermoplastic and thermoset resins must be mutually partially miscible, or mutually miscible between 10 and 60%. The patent also states that the cure temperature does not significantly exceed the glass transition temperature of the thermoplastic resin. At such temperatures the thermoplastic polymer is solid or has an extremely high viscosity, and migration of the uncured thermoset polymer into an amorphous thermoplastic polymer, and formation of a semi-interpenetrating network is quite slow. This is confirmed by the long cure cycles mentioned in the patent.
Further, the invention described in U.S. Pat. No. 5,667,881 relates to the formation of an integral joint with a prefabricated thermoplastic article, which places constraints on the type of article that may be attached using this technique, when compared to the formation of a functional thermoplastic surface.
The present invention advantageously alleviates at least some of the disadvantages of the processes described above, and provides an improved process for forming a thermoplastic surface on a thermosetting polymer or thermosetting polymer composite.
A further advantage of the present invention is an improved process for joining a thermosetting polymer or thermosetting polymer composite component, having a thermoplastic surface, to a second component with a suitable thermoplastic surface.